The invention concerns methods and apparatus for cutting, and more particularly, it relates to forming pellets or granules from extruded lengths of thermoplastic material.
There are numerous types of equipment for cutting thermoplastics. The most common can be schematically described as follows: Thermoplastic material which has been melted within an extruder is forced by the extruder screw through a die provided with cylindrical orifices arranged in one line or more parallel lines or in one or more concentric rings.
In the first instance, the multiple lengths which emerge from the die can be cooled by passing them into a water bath, and solidified. They then pass into a granulator.
In the second type, a cutting head driven by a separate motor rotates a system of blades having rotational symmetry around the axis of the die, and this head cuts up the extruded lengths. The pellets of extruded material obtained are most usually flung by centrifugal force into a casing over the walls of which circulates a film of water at a high flow rate, the water being fed by a circulation pump. One improvement in such cutting apparatus involves immersing the head in a cooling liquid which is maintained in a watertight chamber fitted to the exterior surface of the die.
This chamber is completely filled by the cooling liquid, and the blade-carrying equipment occasions considerable turbulence as it rotates. It is recognized that with such submerged head apparatus, it is very important, in order to obtain the optimum cutting of pellets from the thermoplastic material, that the play between the blades and the external surface of the die be kept to a minimum. If this play becomes too great, for example, as a result of excessive wear of the blades or because of temperature and pressure variations within the material, a mediocre quality cut results, the cut is generally not clean and complete, and the quantity of fines produced increases substantially.